The washing machine-sized projectile is composed of copper because this metal is not expected to appear in the natural chemical signature of the comet itself.

Deep Impact will provide a glimpse beneath the surface of a comet, where material and debris from the time of the Solar System's formation remain relatively unchanged.

Comets are the undercooked leftovers that remained when a sprawling cloud of dust and gas condensed to form the Sun and planets 4.6 billion years ago.

Mission scientists are confident the project will answer basic questions about how the Solar System came to be, by offering a better look at the nature and composition of these frozen balls of ice and rock.

It has been suggested that comets first delivered the basic building blocks of life - carbon-based molecules and water - to Earth.

The energy released by the collision is equivalent to that released by exploding 4.5 tonnes of TNT.

It could create a hole in the comet ranging from the size of a two-bedroom house to the size of a ten-storey building. But the impact is definitely not expected to divert the path of Tempel 1.

The mission was due to launch at the end of December. But during a review, mission scientists and engineers found a few errors, which included a heat ring that had to be replaced.

Deep Impact has a "window" in which it has to launch which lasts until 28 January.

The European Space Agency (Esa) has launched its own mission, Rosetta, which is expected to arrive at the comet Churyumov-Gerasimenko in 2014. It will despatch a lander to study the surface chemistry of the comet nucleus.